1. Field of the Invention
The present invention relates to a pneumatic tire, more particularly to an improved tread pattern suitable for driving on highway as much as off-highway driving.
2. Description of the Related Art
In recent years, 4WD vehicles are a favorite as a town-use car and also 4WD-type recreational vehicles become popular. Thus, such vehicles have a number of occasions for driving on highway as much as off-highway driving.
In order that the 4WD vehicle tire fulfills its original function, namely, traction or road grip on soft terrain, mudy road, snowy road and the like, the tread pattern is desired to have a large grooved area. Conventionally, the tread portion is provided with wide circumferential grooves and wide axial grooves to form generally square tread blocks. However, such tires have disadvantages when driving on highway, that is to say, loud noise is generated during running on the well paved roads.
The noise includes so called: impact sound heard when the tread blocks impact the road surface; pumping sound heard when the air in the center region of the ground contacting patch jets out toward both sides of the tire through the lateral grooves; and resonance sound heard when the air in the wide circumferential groove in the ground contacting patch is excited by the pumping sound and vibrations of the tread rubber and as a result a resonance at a certain frequency is caused.
If the grooved area is decreased and the lateral grooves are inclined, then the noise resulting from the resonance sound, pumping sound and impact sound can be reduced, but wet performance such as aquaplaning resistance is deteriorated. If the size of middle tread blocks, which are disposed in a middle tread region where vibrations are liable to occur, is increased, then the resonance sound excited by the vibrations can be reduced, but wet performance and bad road traction/grip are deteriorated.